1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to a system and method for modulating an input signal with a digital signal modulator and splitting an output signal of the digital signal modulator into multiple output signals.
2. Description of the Related Art
Analog and digital modulators are utilized to convert analog and digital input signals into drive signals. For example, the modulators convert an input signal into discrete pulses using well-known pulse width modulation techniques. The pulses are used as drive signals. The drive signals are utilized to drive output current to a load. In an acoustic application, voice signals may be modulated to drive a load, such as audio speakers.
Power converters may be used to convert direct current (DC) to alternating current (AC) to be used as an AC power supply, or as battery chargers/dischargers, motor controls, etc. Power converters may also be used as amplifiers, both for entertainment (sound amplification) and industrial uses. Many conventional pulse width modulated (PWM) converters use a pair of switches to connect a load alternatively to DC power supplies of opposite polarity. A modulator alternately opens and closes the switches to produce a width modulated output signal that is subsequently filtered by a low pass filter before being transmitted to the load. Care must be taken to assure that both switches are not turned xe2x80x9conxe2x80x9d at the same time to prevent drawing transient xe2x80x9cshoot-throughxe2x80x9d current. Several ways to limit or prevent such shoot-through current have been used. For example, current limiting inductors may be used, or xe2x80x9cunderlapxe2x80x9d circuits may be utilized to create small controlled time gaps between the conduction times of the switches. Opening and closing the switches creates a generally undesirable xe2x80x9cripplexe2x80x9d frequency on an output waveform generated by the conventional modulator.
Opposed current converters (xe2x80x9cOCCsxe2x80x9d) address the problem of ripple frequency generation. U.S. Pat. No. 5,657,219 entitled xe2x80x9cOpposed Current Power Converterxe2x80x9d by Gerald R. Stanley (referred to herein as the xe2x80x9cStanley patentxe2x80x9d) discloses an example of an OCC. Stanley and Bradshaw, Precision DC-to-AC Power Conversion by Optimization of the Output Current Waveform-The Half Bridge Revisited, IEEE Transactions on Power Electronics, Vol. 14, No. 2, March 1999 provide additional discussion on OCCs. OCCs, which include amplifiers referred to as class-I amplifiers, opposed current amplifiers, balanced current amplifiers, and opposed current interleaved amplifiers, are particularly useful in audio applications due to their high efficiency and high signal to noise ratios in frequency bandwidths of interest.
Referring to FIG. 1, the Stanley patent discloses a power converter circuit 100, which is also sometimes referred to as an opposed current amplifier stage. Power converter circuit 100 receives two input drive signals Spxe2x80x2 and Snxe2x80x2. Signals Spxe2x80x2 and Snxe2x80x2 are square-waves with pulse-widths that are determined by modulating an input signal.
Power converter circuit 100 has four states of operation in the continuous current mode. Signals Spxe2x80x2 and Snxe2x80x2 determine the states of operation by respectively controlling the conductivity of switches 102 and 104. Switches 102 and 104 conduct during the interval when Spxe2x80x2 and Snxe2x80x2 are both HIGH causing the main output inductor currents Ip and In to increase at a rate of approximately V/L, in which L=Lp=Ln and V is the magnitude of each supply voltage (Vsupply). When Spxe2x80x2 and Snxe2x80x2 are both HIGH, the magnetization of inductors Lp and Ln are increased. When Spxe2x80x2 and Snxe2x80x2 are both LOW, switches 102 and 104 become nonconductive, the inductor voltages are reversed, the diodes 108 and 110 conduct, and the inductor current magnitudes ramp down at the same rate. When Spxe2x80x2 and Snxe2x80x2 are both LOW, the magnetization of inductors Lp and Ln are decreased. When Spxe2x80x2 is LOW and Snxe2x80x2 is HIGH, switch 104 and diode 110 conduct resulting in negative output current (lout) into output node 106. When Spxe2x80x2 is HIGH and Snxe2x80x2 is LOW, switch 102 and diode 108 conduct resulting in positive output current lout from node 106.
Table 1 summarizes the four continuous current mode states of operation for power converter circuit 100 with reference to signals Spxe2x80x2 and Snxe2x80x2. Table 1 uses xe2x80x9cHIGHxe2x80x9d and xe2x80x9cLOWxe2x80x9d to represent the states of signals Spxe2x80x2 and Snxe2x80x2. In the embodiment of FIG. 1, a HIGH signal causes switches 102 and 104 to conduct, and a LOW signal causes switches 102 and 104 to open.
Referring to FIG. 2, the Stanley patent describes an analog modulator 200 utilized to produce signals Spxe2x80x2 and Snxe2x80x2 for drive power converter circuit 100. Analog modulator 200 utilizes an error amplifier 202 to generate an error signal 204 from an input signal 206, representing a desired level at output node 106 of power converter circuit 100, and a feedback signal 208 received from output node 106. Inverter 218 inverts error signal 204 to generate inverse error signal 216. Comparators 210 and 214 generate respective signals Spxe2x80x2 and Snxe2x80x2 by comparing a triangle waveform 212 with respective error signal 204 and inverse error signal 216. Signal Spxe2x80x2 is HIGH when the magnitude of triangle waveform 212 exceeds error signal 204, and signal Spxe2x80x2 is LOW when the magnitude of error signal 204 exceeds triangle waveform 212. Likewise, signal Snxe2x80x2 is HIGH when the magnitude of triangle waveform 212 exceeds inverse error signal 216, and signal Snxe2x80x2 is LOW when the magnitude of inverse error signal 214 exceeds triangle waveform 212.
For example, the triangle waveform 212 is also biased to address cross-over distortion during the switching of switches 102 and 104. Triangle waveform generator 220 generates triangle waveform 212 from a square wave input signal 222. The direct current (DC) level of the triangle waveform 212 is adjusted by adding or subtracting bias signal 224 from triangle waveform 212. The bias is normally adjusted such that, at input signal equal zero, both switches are on slightly more than fifty percent (50%) of the time, and an idle current exists in the inductors Ln and Lp which keeps the diodes 108 and 110 clamped during the de-magnetization phase.
Referring to FIG. 3, U.S. Pat. No. 6,373,336, entitled Method of Attenuating Zero Crossing Distortion and Noise in an Amplifier, an Amplifier and Uses of the Method and the Amplifier, inventors Niels Anderskouv and Lars Risbo, (referred to herein as the xe2x80x9cAnderskouv-Risbo patentxe2x80x9d) describes an example of an amplifier 300 using dual pulse width modulators, PWM A and B, to drive respective half bridge amplifiers A and B connected to load 302. A signal source 304 provides an input signal to inverting block 306 and noninverting block 308. PWM A provides one output signal to drive the switches of Half bridge A, and PWM B provides one output signal to drive the switches of Half bridge B. The Anderskouv-Risbo patent introduces a delay element AT into the signal path between PWM B and half bridge B to prevent simultaneous switching of switches on the half bridges A and B and, thus, attenuate cross-over distortion. The Anderskouv-Risbo patent does not teach providing appropriate signals to drive switches separately within a half bridge amplifier such as power converter circuit 100. In contrast, the Stanley patent teaches switching techniques for use within a half bridge. Two copies of the Stanley circuit could be used for creating a full bridge circuit.
For example, the Anderskouv-Risbo patent and other references do not address the application of digital signal processing technology to provide appropriate input signals to loads such as power converter circuit 100.
In embodiments of the present invention, a digital input signal is modulated using a digital signal modulator to provide multiple signals to drive a load, such as an opposed current converter (OCC).
In one embodiment of the present invention, an apparatus includes a first digital signal modulator to generate a first modulated output signal derived from a digital input signal. First duty cycle circuitry, coupled to the first digital signal modulator, to receive first and second input signals, which are respective subsets of samples of the first modulated output signal. The first duty cycle circuitry responds to the first and second input signals and respectively generates a first output signal and a second output signal. In one embodiment, the first digital signal modulator includes a delta-sigma modulator, and the first duty cycle circuitry includes two pulse width modulators. During operation of the first digital signal modulator and the first duty cycle circuitry, a duty cycle of the first output signal has a direct relationship to change in the digital input signal, a duty cycle of the second output signal has an inverse relationship to change in the digital input signal. The first and second output signals of the second circuitry are suitable for driving an opposed current converter stage.
In another embodiment of the present invention, a method of providing multiple output drive signals derived from a common input signal includes receiving a common digital input signal and a first digital bias signal. The common input signal is a digital audio signal, and the first digital bias signal is derived from even numbered samples of a bias signal. The method further includes converting the common digital input signal and the first digital bias signal into a first output signal using a first modulator. The method also includes receiving the common digital input signal and a second digital bias signal and converting the common digital input signal and the second digital bias signal into a second output signal using the first modulator. The second digital bias signal is derived from odd numbered samples of the bias signal. The method further includes providing the first and second output signals to circuitry operable to derive a drive signal from the first and second output signals. In one embodiment, the drive signal is derived by using the first and second output signals to generate the drive signal of an opposed current converter.